The present invention relates to a novel tundish useful in the casting of a continuous ribbon of material, e.g., semiconductor material.
More particularly, this invention concerns a tundish having a support for a hopper containing molten material, e.g., semiconductor material, the hopper having a bottom plate and three sidewall plates and a top plate, each made of a material which will not interact with the molten semiconductor material contained within the space defined by the bottom plate, sidewall plates and the top plate, to impart any significant impurity into the molten material, and wherein the top plate slants downwardly towards the bottom plate to form an elongated orifice through which molten material is extruded onto a cool surface where the crystallization of a ribbon of the material occurs.
The invention also relates to the top plate, bottom plate, or both being movable with respect to the other to change the thickness of the orifice, with the top plate and/or bottom plate being slideably mounted in a slot in each of two opposed sidewalls and the top plate being of such a length that the top plate forms, with the third sidewall, an opening into which molten material is added to the tundish.
Ribbon casting of a ribbon of crystalline and/or coarse grain polycrystalline material, e.g., semicondoctor material, occurs when a thin ribbon of molten material is forced or extruded onto a cool surface, which is usually moving with respect to the source point of extrusion of the molten material. Usually this is done by forcing or extruding the semiconductor material onto a rotating drum which is cooled so that crystallization will occur while the ribbon is in contact with the surface of the drum.
It is desirable to have a tundish which will supply the molten material to the surface of the rotating drum in a thin ribbon which is adjustable to a variety of desired thicknesses at the point of extrusion. This enables the precise setting of the thickness of the crystallized ribbon of the material, which is determined by the thickness of the extruded molten ribbon of material and the amount of stretch of the ribbon between the orifice and the cooled surface, which in turn depends on the type of material, its temperature, the distance between the orifice and the point of contact on the cooled surface and the speed of rotation of the cooled surface of the drum. It is also desirable that the material of the tundish be such that it will not interact with the molten material to impart any impurity to the molten material. This is particularly true when the molten material is semiconductor material being cast into a ribbon for use in, e.g., a photovoltaic cell, where even slight amounts of impurities in the semiconductor material can cause a significant decrease in the efficiency of the cell.
It is also desirable that the tundish contain a heating element to maintain the temperature of the molten material above the temperature at which crystallization of the molten material will commence and also to heat the orifice of the tundish through which the ribbon of molten material is extruded in order to insure that the molten material is extruded onto the rotating drum in a molten state.
The problems enumerated above which must be overcome in providing an adequate tundish for ribbon casting of crystalline or polycrystalline material, are not intended to be exhaustive, but, rather, are among many which tend to impair the effectiveness of previously known methods or apparatus for supplying molten material to the surface of the rotating drum. Other noteworthy problems may also exist, however, those presented above should be sufficient to demonstrate that there is a need appearing in the art for an improved apparatus for supplying molten material to the rotating drum surface.